1. Field of the Invention
The present invention relates to optical communications and more particularly to a waveguide-type wavelength mul-tiplexing optical transmitter/receiver module used for wavelength multiplexing transmission.
2. Description of the Related Art
In recent years, the need for bidirectional optical communications has increased and the adaptation of this system to home use has long been desired as well. To establish bidirectional optical communications, an optical transmitter and an optical receiver are required. However, the, separate or discrete construction of these two devices inevitably causes upsizing of an optical transmitting and receiving system, hindering the widespread use thereof. Furthermore, it is not economical to lay two optical fibers for sending and receiving for home use; and therefore, it is essential to establish bidirectional communications by employing only one optical fiber.
To do communications with one optical fiber, a device such as an optical transmitter and receiver intended for the home which can be connected to one optical fiber is required.
At this point, it is necessary to isolate an upward signal light from individual households from a downward signal light from a station.
For isolating the upward signal light from the downward signal light, there is a method in which two types of light each having a different wavelength are used. Accordingly, an optical demultiplexer is required which is able to differentiate a wavelength of light signal outputted from a light transmitter from that inputted to a light receiver. In addition, the device having such functions must be small-sized and low-priced for individual households. To meet these requirements, the integration of an optical transmitter, receiver and de-multiplexer into one device is essential. With this in mind, for the purpose of making the device small-sized, highly-integrated and low-priced, the use of an optical waveguide has been investigated. A silica glass waveguide in particular has been generally employed owing to its small propagation loss and excellent coupling efficiency to an optical fiber. Though the silica glass waveguide itself has no property of sending or receiving light, as a method for highly efficient coupling to a laser diode (LD) being an optical transmitter or a photodiode (PD) being an optical receiver has been recently developed; its importance as an optically packaged substrate is increasingly recognized. In the past, the coupling of the LD to the optical waveguide was impossible due to a very big coupling loss caused by a difference in spot size. However, the LD allowing changes in its spot size has been recently developed, providing a high efficiency in coupling to the optical waveguide.
In the case of a PD, owing to its relatively large light-receiving area of about 80-100 xcexcm, the coupling to an optical waveguide can be easily implemented.
An optical transmitter/receiver module having an optical demultiplexer composed of an optical waveguide disclosed in Japanese Laid-open Patent Application No. Hei 9-105824 has been known in the art.
FIG. 11 shows the conventional waveguide-type wave-length multiplexing optical transmitter/receiver module described in the above Japanese Provisional Publication.
As depicted in FIG. 11, if there is light with two types of wavelengths, for example, xcex1 and xcex2, a dielectric multilayer filter 108 is so configured as to allow the light with one wavelength to pass therethrough and reflect the light with the other wavelength. The demultiplexing function of this module has been achieved by providing, using:a dicing process, a groove 107 in optical waveguides 110, 111 and 112 formed on a substrate 109 and by inserting the dielectric multilayer filter 108 into the groove 107.
The light with a wavelength of xcex2 from a port 101 passes through the dielectric multilayer filter 108 and is outputted from a port 103 while the light with a wavelength of xcex1 from a port 102 reflects off of the dielectric multilayer filter 108 and is outputted from the port 101.
Thus, the wavelength multiplexing optical transmitter/receiver module has been implemented by connecting an optical fiber 104 to the port 101 of the optical demultiplexer and by connecting each of the LD 105 and PD 106 to a port 102 and 103 respectively.
An optical transmitter/receiver module having an optical demultiplexer composed of an optical waveguide disclosed in Japanese Laid-Open Patent Application No. Hei 09-159851 has been known in the art as another example.
FIG. 12 shows the conventional waveguide-type wavelength multiplexing optical transmitter/receiver module described in the above Japanese Provisional Publication.
Referring to FIG. 12, a dielectric multilayer filter 208 has properties of reflecting light of a wavelength of xcex1 and allowing light of a wavelength of xcex2 to pass therethrough.
Two optical waveguides 210 and 211 are formed on a substrate 209, and one end of the optical waveguide 211 is connected to an optical fiber 204 on a port 201 while one end of the optical waveguide 210 is connected to a LD 5 at a port 202.
The other end of each of the optical waveguides 210 and 211 are positioned so that they converge at an end of the substrate 209 to constitute a directional coupler 212.
The directional coupler 212 is adapted to provide one-half coupling length of a perfect coupling length to light with a wavelength of xcex1 outputted from a LD 205 connected to one end of the optical waveguide 210.
The demultiplexing function of this module has been thus achieved by affixing the dielectric multilayer filter 208 through a transparent substrate 213 to an end face of the side of the substrate 209 where the directional coupler 212 is formed thereon.
The light with a wavelength of xcex1 outputted from the LD 205 propagates through the optical waveguide 210 and, after entering the directional coupler 212, reflects off this dielectric multilayer filter 208 and is outputted to the optical waveguide 211 and then eventually enters the optical fiber 204. On the other hand, the light with a wavelength of xcex2 from the optical fiber 204 propagates through the optical waveguide 211 and, after entering the directional coupler 212, passes through the dielectric multilayer filter 208 and enters the PD 206 positioned in a backward position and then eventually is received.
However, there are problems to be solved in the conventional module. In the conventional wavelength multiplexing optical transmitter/receiver module having an optical demultiplexer wherein the dielectric multilayer filter as described above is positioned on the optical waveguide, the module has a high function to prevent light with a wavelength of xcex1 from the LD 105 from entering the PD 106 (206) by reflecting the light off the dielectric multilayer filter 108 (208). However, the light with a wavelength of xcex2 or so from the LD 105, though being generally weak, passes through the dielectric multilayer filter 108 (208), causing all the light to be received by the PD 106 (206).
According to experiments, for example, in the case of a level of an output with a wavelength of xcex1 from the LD 105 being 0 dBm (i.e., 1 mW), even if a dielectric multilayer filter is used which is capable of isolating light with a wavelength of xcex1 at a level of xe2x88x9250 dB or so, the output level of xe2x88x9240 dBm has been detected at the PD 106(206).
Generally, light receiving characteristics of a photodiode in an optical transmitter/receiver module are determined depending on how low the level of signal light from an optical fiber can be received.
For example, if the level of to-be-received signal light transmitted from an optical fiber is xe2x88x9240 dBm, the level is almost the same as that of to-be-sent signal light transmitted from a LD, making it impossible to differentiate to-be-received light from to-be-sent light.
The level of to-be-received signal light actually transmitted from an optical fiber is about xe2x88x9230 dBm and the to-be-sent signal light transmitted to the PD 106(206) turns out to be a noise, hindering good reception of the signal light.
Accordingly, the optical transmitter/receiver module having such configurations as disclosed in the conventional technology cannot provide good light receiving characteristics and has presented a problem in that the conventional system is difficult to apply to practical use.
In view of the above, it is an object of the present invention to provide a waveguide-type wavelength multiplexing optical transmitter/receiver module having simplified and low-priced configurations and excellent characteristics.
According to a first aspect of the present invention, there is provided a waveguide-type wavelength multiplexing optical transmitter/receiver module comprising:
an optical demultiplexer;
an optical transmitter;
an optical receiver;
the optical demultiplexer including an optical waveguide substrate and a filter;
whereby the optical waveguide substrate having a first optical waveguide and a second optical waveguide and a cross section wherein the first optical waveguide and the second waveguide cross each other on a first end face of the optical waveguide substrate, the first optical waveguide and the second optical waveguide being disposed in a position opposite to each other relative to a cross axis being normal to the first end face and passing through the cross section, the first optical waveguide having a first port connected to the optical transmitter on a second end of said optical waveguide substrate, the second optical waveguide having a second port connected to an optical fiber;
the filter being disposed on the first end face of the optical waveguide substrate so as to reflect light with a first wavelength from the optical transmitter and to send it to the second optical waveguide and to allow light with a second wavelength from the optical fiber to pass through; and
the optical receiver the center of which is disposed in a shifted position toward the direction of the first optical waveguide relative to the cross axis so as to receive light with a second wavelength passed through the filter.
In the foregoing, a preferable mode is one wherein the optical receiver is disposed so that the location of said center axis is shifted in parallel relative to the cross axis and in the direction of the optical waveguide.
Also, a preferable mode is one wherein the optical receiver is disposed so that the center axis of the optical receiver is in a rotated position in the direction of the first optical waveguide.
Also, a preferable mode is one wherein the optical demultiplexer has a unwanted light removal filter for allowing light with first wavelength from the optical transmitter to pass therethrough and for removing an disposed light.
Furthermore, a preferable mode is one wherein the unwanted light removal filter is interposed between the first port of the first optical waveguide and the cross section.
According to a second aspect of the present invention, there is provided a waveguide-type wavelength multiplexing optical transmitter/receiver module comprising:
an optical demultiplexer;
an optical transmitter;
an optical receiver;
the optical demultiplexer including an optical waveguide substrate and a filter;
the optical waveguide substrate having a first optical waveguide and a second optical waveguide and a cross section wherein the first optical waveguide and the second optical waveguide converge on the first end of the optical waveguide substrate;
a directional coupler adapted to provide one-half coupling length of perfect coupling length to light with a first wavelength from the optical transmitter;
the first optical waveguide having a first port for connecting to the optical transmitter on the second end face of the optical waveguide substrate;
the second optical waveguide having second port for connecting to an optical fiber;
an unwanted light removal filter being somewhere on said first optical waveguide and being disposed between the first port and the directional coupler for allowing light with a first wavelength from the optical transmitter to pass through the filter and removing unwanted light, the filter disposed on first end face of said optical waveguide substrate for reflecting light with a first wavelength from the optical transmitter to send it to the second optical waveguide and for allowing light of a second wavelength from the optical fiber.
In the foregoing, it is preferable that the filter is a dielectric multilayer filter.
Also, it is preferable that the unwanted light removal filter is composed of a directional coupler.
Also, it is preferable that the unwanted light removal filter is composed of a Mach-Zehnder interferometer.
Also, it is preferable the unwanted light removal filter is composed of a dielectric multilayer filter.
A preferable mode is one wherein the unwanted light removal filter is implemented by inserting a dielectric multilayer filter into a groove formed across the first optical waveguide of the optical waveguide substrate.
A preferable mode is one wherein the optical waveguide substrate consists of one of semiconductor compounds including LiNbO3(lithium niobate), Si(silicon), GaAs (gallium arsenide), InP(indium phosphide) substrates.
Also, a preferable mode is wherein said optical transmitter is a laser diode.
Furthermore, a mode is preferable wherein the optical receiver is a photodiode.